Morphology and Wall Ultrastructure in Devonian Spores with Bifurcate‐Tipped Processes

Abstract

A detailed investigation has been undertaken on two dispersed spore taxa, characterized by their zonate structure and an ornament of bifurcate‐tipped processes, that were recovered from Middle Devonian deposits from Scotland: Ancyrospora grandispinosa Richardson 1960 and Ancyrospora (Triletes) ancyrea (Eisenack) Richardson 1962. Both taxa have been comprehensively examined using light, scanning electron, and transmission electron microscopy, and a detailed understanding of their morphology, gross structure, and wall ultrastructure has emerged. Both have an inner body, enclosed within an outer layer that is extended into a pseudozona, and bifurcate‐tipped processes of similar construction. On the whole, morphology and wall ultrastructure are strikingly similar in the two species, although subtle differences lead to slight reservations when attempting to identify homologous wall layers. Affinity of the spores is assessed based on detailed comparisons with the spores of plant groups believed to have comprised the Middle Devonian vegetation. I conclude that the taxa are closely related and have lycopsid affinities, the most compelling evidence for the latter being similarities in wall ultrastructure with extant and fossil lycopsid spores. Mode of spore wall formation is interpreted in light of the new data. Because of the absence of ontogenetic information, I acknowledge the speculative nature of my interpretation. However, it is clear that in both taxa, the bifurcate‐tipped processes are constructed in a similar manner, and this involves a relatively simple developmental process. Spores with bifurcate‐tipped processes are a conspicuous, but enigmatic, feature of Middle and Upper Devonian spore assemblages. Their reproductive strategies, functional morphology, and paleoecology are discussed in light of the new data.